PVdF is recognized as exhibiting valuable properties which make its use as a binding agent very desirable. It is stable towards aggressive chemicals; and it is thermally stable, dirt-repellent, scratch resistant, weather-resistant and is not readily broken down by UV radiation. The durability of PVdF is evidenced by its tough finish as well as its ability to resist crazing and chalking with changing weather conditions.
Because of these advantageous properties, PVdF-based coatings are known to be very useful as protective coatings for a variety of substrates, more particularly for building panels and other articles made from metal sheets intended for outdoor use. However, while PVdF-based compositions provide a very satisfactory topcoating, they adhere poorly to metal substrates and have less than satisfactory corrosion resistance. Thus, PVdF-based coatings usually need to be applied on a suitable primer coating, rather than directly to the metal, mainly for improving adhesion to the substrate but also in order to improve corrosion resistance of the substrate. This is particularly necessary with metal substrates such as steel and aluminum. For this reason, many different primer coating compositions have been formulated. The composition of the primer depends, among other factors, on the topcoat which will subsequently be applied to it.
As primer for PVdF topcoats, it has already been proposed to use compositions of an acrylic resin, an epoxy resin and an organosilicon or organoaluminum compound. However, these compositions require the use of compounds which are designed to improve the adhesion to the substrate and are therefore delicate to use.
European Patent Application EP-A-294755 discloses a PVdF-based liquid coating product, based on a solvent and a ternary polymer mixture of PVdF, a second polymer incompatible therewith, and a compatibilizer, which product is be used as topcoat without primer coating. However, the preferred choice of an epoxy resin as the incompatible polymer considerably reduces the weathering resistance of the resulting coating.
Although the known wet systems may give good results, environmental protection laws throughout the world, but mainly in the U.S.A. and Europe, are making it increasingly difficult to handle solvent-based coating systems. Further, the recovery of the solvent is a costly procedure. Still further, use of both a powder top coating and a wet primer system requires two separate sets of application equipment, thus increasing fixed costs. Accordingly, there is a significant need for solventless powder compositions for primer coatings.
However, it is difficult to find a primer composition for PVdF powder coatings that will yield a system having good properties. Accordingly, there is a significant need in the art for improved solvent-free primer coatings for PVdF-based powder coatings, said primer coatings being capable of adhering well to a metal substrate as well as having good intercoat adhesion with the PVdF-based topcoats, while providing good corrosion resistance to the substrate. Additionally, the primer itself must retain its integrity, i.e. neither craze nor deteriorate.